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Slide 1 Back Chapter 16-Acids and Bases Chapter 16 Acid-Base Equilibria AP CHEMISTRY Content from The Central Science, 9 th ed David P. White, revised.

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Presentation on theme: "Slide 1 Back Chapter 16-Acids and Bases Chapter 16 Acid-Base Equilibria AP CHEMISTRY Content from The Central Science, 9 th ed David P. White, revised."— Presentation transcript:

1 Slide 1 Back Chapter 16-Acids and Bases Chapter 16 Acid-Base Equilibria AP CHEMISTRY Content from The Central Science, 9 th ed David P. White, revised 2004 by CMB

2 Slide 2 Back Chapter 16-Acids and Bases Acids: taste sour and cause dyes to change color. Bases: taste bitter and feel soapy. Arrhenius Definition: acids increase [H + ] bases increase [OH - ] in solution. Arrhenius: acid + base salt + water. Arrhenius limits us to aqueous solution. Acids and Bases: A Brief Review

3 Slide 3 Back Chapter 16-Acids and Bases Hydronium: The H + Ion in Water The H + (aq) ion is simply a proton with no electrons. Generally we use H + (aq) and H 3 O + (aq) interchangeably. Brønsted-Lowry: acid donates H + and base accepts H +. Brønsted-Lowry base does not need to contain OH -. Consider HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) : HCl donates a proton to water. Therefore, HCl is an acid. H 2 O accepts a proton from HCl. Therefore, H 2 O is a base. Water can behave as either an acid or a base-its amphoteric. Amphoteric substances can behave as acids and bases. Brønsted-Lowry Acids and Bases Example on next page

4 HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) :

5 Slide 5 Back Chapter 16-Acids and Bases The Ion Product of Water In pure water the following equilibrium is established at 25 C The above is called the autoionization of water. In neutral, pure water, [H 3 O + ] = 1 x M In neutral, pure water, [OH - ] = 1 x M The Autoionization of Water

6 Slide 6 Back Chapter 16-Acids and Bases p = -log In most solutions [H + (aq)] is quite small. We define In neutral water at 25 C, pH = pOH = In acidic solutions, [H + ] > , so pH < In basic solutions, [H + ] The higher the pH, the lower the pOH, the more basic the solution. There are no theoretical limits on the values of pH or pOH. (e.g. pH of 2.0 M HCl is ) The pH Scale Logarithm algebra is needed here

7 Slide 7 Back Chapter 16-Acids and Bases pH meter – measures conductivity as a function of [H 3 O+]. Precise, but calibration is often needed for accuracy Indicators – dyes that change their bonding & color as a function of [H 3 O+]. Less precise than pH meters, but quick and cheap Most indicators tend to be red in more acidic solutions. The pH Scale, & Measuring pH

8 Indicators & the pH Scale Transition Interval: the range of pH that they are effective. Endpoint: the pH that produces a noticeable color change

9 Slide 9 Back Chapter 16-Acids and Bases Definition Conjugate Base = Whatever is left of the acid after the proton is donated. Conjugate Acid = Whatever remains of the base after it accepts a proton. Consider –After HA (acid) loses its proton it is converted into A - (base). Therefore HA and A - are conjugate acid-base pairs. –After H 2 O (base) gains a proton it is converted into H 3 O + (acid). Therefore, H 2 O and H 3 O + are conjugate acid-base pairs. Conjugate acid-base pairs differ by only one proton. Conjugate Acid-Base Pairs

10 Slide 10 Back Chapter 16-Acids and Bases Identify Conjugate pairs: Acid Base Conjugate Acid Conjugate Base Conjugate Acid

11 Slide 11 Back Chapter 16-Acids and Bases Relative Strengths of Acids and Bases Strengths of Acids or Bases is determined by the extent to which a species dissociates, or ionizes in water.

12 Slide 12 Back Chapter 16-Acids and Bases Rules Rules: Relative Strengths of Acids and Bases The stronger the acid, the weaker the conjugate base. The stronger the base, the weaker the conjugate acid. A strong acids conjugate base (e.g. Cl - ) has negligible acid-base properties. A strong bases conjugate acid has negligible acid-base properties. Conjugates of weak acids are weak bases. Conjugates of weak bases are weak acids. Acid strength increases across a period and down a group. Base strength decreases across a period and down a group. Conjugate Acids and Base Strength

13 Slide 13 Back Chapter 16-Acids and Bases Strong Acids and Bases *MEMORIZE* Six Strong Acids*: HClO 4 HI HBr HCl H 2 SO 4 HNO 3 HNO 3(aq) H + (aq) + NO 3 - (aq) Strong Bases: NaOH KOH Ba(OH) 2 etc-OH (those soluble) Strong bases need not have OH - (e.g.: H - (aq) + H 2 O (l) H 2(g) + OH - (aq) ) All others, consider to be weak (not completely dissociated) Strong acids & bases are strong electrolytes. Weak acids and bases = Ions are in equilibrium with unionized compounds. *Your book: HClO 3 is also strong, but not on AP test.

14 Slide 14 Back Chapter 16-Acids and Bases The of the acid-base on the AP Weak acids are only partially ionized in solution. Therefore, weak acids are in equilibrium: K a is the acid dissociation constant. The larger the K a the stronger the acid (i.e. the more ions are present at equilibrium relative to unionized molecules). If K a >> 1, then the acid is completely ionized and the acid is a strong acid. Weak Acids

15 Slide 15 Back Chapter 16-Acids and Bases Some Weak Acids:

16 Slide 16 Back Chapter 16-Acids and Bases Weak acids are simply equilibrium calculations. The pH gives the equilibrium concentration of H +. Using K a, the concentration of H + (and hence the pH) can be calculated: 1.Write the balanced chemical equation clearly showing the equilibrium. 2.Write the equilibrium expression. Find the value for K a. 3.Write down the initial and equilibrium concentrations for everything except pure water. We usually assume that the change in concentration of H + is x. 4.Substitute into the equilibrium constant expression and solve. Remember to turn x (at equilibrium) into pH if necessary. We can also solve for K a from pH or [H 3 O + ] in this manner. Calculating pH or [H 3 O + ] from K a

17 Slide 17 Back Chapter 16-Acids and Bases Using %Ionization to Calculate pH or Ka Percent ionization is another method to assess acid strength. For the reaction The higher percent ionization, the stronger the acid. Percent ionization of a weak acid decreases as the molarity of the solution increases. For acetic acid, 0.05 M solution is 2.0 % ionized whereas a 0.15 M solution is 1.0 % ionized. % Ionization

18 Slide 18 Back Chapter 16-Acids and Bases Polyprotic Acids have more than one ionizable H + The protons are removed in steps not all at once: It is always easier to remove the first proton in a polyprotic acid than the second. Therefore K a1 > K a2 > K a3 etc. Polyprotic Acids

19 Slide 19 Back Chapter 16-Acids and Bases Weak bases remove protons from substances. There is an equilibrium between the base and the resulting ions: Example: The base dissociation constant, K b, is defined as Weak Bases

20 Slide 20 Back Chapter 16-Acids and Bases Trends in Weak Bases Bases generally have lone pairs or negative charges in order to attack protons. Most neutral weak bases contain nitrogen. Amines are related to ammonia and have one or more N-H bonds replaced with N-C bonds (e.g., CH 3 NH 2 is methylamine). Anions of weak acids are also weak bases. Example: OCl - is the conjugate base of HOCl (weak acid): Weak Bases

21 Slide 21 Back Chapter 16-Acids and Bases Some Weak Bases

22 Slide 22 Back Chapter 16-Acids and Bases When two reactions are added to give a third, the equilibrium constant for the third reaction is the product of the equilibrium constants for the first two: For a conjugate acid-base pair: The larger the K a, the smaller the K b. That is, the stronger the acid, the weaker the conjugate base. Taking negative logarithms: Conjugate Acid/Base Strengths

23 Slide 23 Back Chapter 16-Acids and Bases Ionic salts dissociate completely Acid-base properties of salts are a consequence of the reaction of their ions in solution. The reaction in which ions produce H + or OH - in water is called hydrolysis. Anions from strong acids are neutral. Anions from weak acids are basic: Polyatomic cations with ionizable protons can be considered conjugate acids of weak bases: Acid-Base Properties of Salt Solutions

24 Slide 24 Back Chapter 16-Acids and Bases Consider H-X: For this substance to be an acid we need: H-X bond to be polar H-X bond must be weak enough to be broken The conjugate base, X -, must be stable HF is a weak acid because the bond energy is high and not easily broken. H-I is a strong acid, easy to break H-I bond. Acid strength increases across a period and down a group. Base strength decreases across a period and down a group. Binary Acid Strengths

25 Slide 25 Back Chapter 16-Acids and Bases Binary Acids Acid-Base Behavior and Chemical Structure

26 Slide 26 Back Chapter 16-Acids and Bases Oxyacids contain O-H bonds All oxyacids have the general structure Y-O-H. The strength of the acid depends on Y and the relative location of the electron density near Y: –If Y is a metal (low electronegativity), then the substance is a base. –If Y has intermediate electronegativity (e.g. I, EN = 2.5), the substance is a weak oxyacid. –If Y has a large electronegativity (e.g. Cl, EN = 3.0), the electrons are located closer to Y than O and the O-H bond is polarized to lose H +. –More O atoms attached to Y increase the O-H bond polarity and the strength of the acid increases (e.g. HOCl is a weaker acid than HClO 2 which is weaker than HClO 3 which is weaker than HClO 4 which is a strong acid). Oxy-acids

27 Slide 27 Back Chapter 16-Acids and Bases Oxyacids Acid-Base Behavior and Chemical Structure

28 Slide 28 Back Chapter 16-Acids and Bases Carboxylic Acids Carboxylic acids all contain the COOH group. All carboxylic acids are weak acids. When the carboxylic acid loses a proton, it generate the carboxylate anion, COO -. Acid-Base Behavior and Chemical Structure

29 Slide 29 Back Chapter 16-Acids and Bases Recall: Brønsted-Lowry acid is a proton donor. Focusing on electrons: a Brønsted-Lowry acid can be considered as an electron pair acceptor. Lewis acid: electron pair acceptor. Lewis base: electron pair donor. Note: Lewis acids and bases do not need to contain protons. Lewis acids generally have an incomplete octet (e.g. BF 3 ). Transition metal ions are generally Lewis acids. Lewis acids must have a vacant orbital (into which the electron pairs can be donated). Lewis Acids and Bases

30 Slide 30 Back Chapter 16-Acids and Bases Hydrolysis of Metal Ions (can be tricky) Alkali and Alkaline-earth metals form bases in solution. Transition metal ions are positively charged and attract water molecules (via the lone pairs on O). The higher the charge, the smaller the metal ion and the stronger the M-OH 2 interaction. Hydrated metal ions (water-complexes) act as weak acids: [H 3 O + ] increases as the size of the ion decreases (Ca 2+ / Zn 2+ ) and as the charge increases (Na + / Ca 2+ and Zn 2+ / Al 3+ ). Na + = no acidic behavior, Al 3+ = very acidic cation Metals and pH

31 Slide 31 Back Chapter 16-Acids and Bases Hydrolysis of Metal Ions Lewis Acids and Bases End of chapter 16 (phew)


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